A common use of mechanical arms in dentistry is for supporting dental instruments within reach of the dentist. A tray also may be held by the arm near the instruments. The arm usually includes a brake mechanism for locking the arm in a selected position to hold the tray and instruments at an elevation desired by the dentist. When the brake is released, the tray-holding end of the arm may be raised or lowered as desired.
The mechanical arm includes an elongated arm member that has each end pivotally fastened to a post. One post may be designated as an end post. The other post may be designated a support post that supports a structure for mounting the tray or equipment to the arm. The pivotal motion of the arm about the end post is in a generally vertical direction, although the end post may also be mounted for rotation about a generally vertical axis, thereby providing a generally horizontal motion for the arm. The tray and instruments may be mounted to the support post to be rotated about a vertical axis.
The mechanical arm also includes a rod member that is also pivotally connected at one end to the support post and at the other end to the end post. The rod member and arm member are pinned in generally parallel orientation. The rod member and arm member provide, in conjunction with the posts, a parallel linkage assembly so that as the arm is pivoted about the end post, the support post will retain its orientation relative to vertical, and the tray and instruments will remain horizontal throughout the pivotal motion of the arm.
The rod member may include an intermediate spring portion that is assembled over the rod between the end post and the rod member to counter the weight of the arm system. Accordingly, the spring portion prevents the arm from falling under its own weight whenever the brake is released.
In the past, the construction of a mechanical arm such as just described required small manufacturing tolerances for aligning the pivot axes in the arm and rod member with the axes in the posts. Proper alignment is important for proper operation of the parallel linkage assembly. The pivot axes are established in the arm components by the precise location of holes that receive the pivot pins.